Halftoning is performed to render continuous tone (contone) digital grayscale and color images into patterns of pixels that can be displayed on bi-level devices such as printers. The rendered images are commonly referred to as halftone images. The pixels of a halftone image are arranged in patterns such that the halftone image is perceived as having continuous tones when viewed through the human visual system (HVS).
One method of halftoning involves the use of dither matrices, also known as halftone screens. A halftone screen includes a two-dimensional array of thresholds. Each threshold is a number having a value v ranging from one to M−1, where M represents the total number of gray levels within the gray scale range being used. If the gray scale range has 256 gray levels, the value v is between one and 255. That is, 1≦v≦255.
For standard color dithering, one monochrome screen is used per color separation. In CMYK color space, for instance, a first screen is used for cyan (C), a second screen for magenta (M), a third screen for yellow (Y), and a fourth screen for black (K). Each monochrome screen is applied to a color separation independently of the other screens.
During halftone dithering of a color plane, pixel values in the contone image are compared to thresholds in the halftone screen. If the pixel value in the contone gray scale image pixel has a value larger than or equal to the corresponding threshold, a dot is formed in the corresponding position of the halftone image (assuming an ascending gray level numbering convention is employed, i.e., where higher gray level numbers correspond to darker gray levels). Decisions to place dots are made on a pixel-by-pixel basis.
The halftone images may contain halftone patterns composed of isolated pixels. However, isolated pixels are not reliably reproduced by laser printers, digital printing presses, and other electro-photographic systems.
The halftone images may contain clusters of pixels. The pattern power spectra of the clustered pixels exhibits a strong mid-frequency component, as opposed to the strong high frequency component exhibited by the isolated pixel halftone patterns. Cluster dot screens produce dot clusters, which can be reliably reproduced by electro-photographic devices.
The halftone images can contain harmonic artifacts that appear as parallel lines that are equally spaced apart. The parallel lines might be rotated at an angle. These artifacts can degrade image quality.
It is desirable to reduce the visibility of these harmonic artifacts in halftone images.